Recently, I was on a public book launch connected with an intellectual debate. There were many famous people whose face I know from the news, many of them I very much respect but have no actual connection with. Naturally, they are engaged in a conversation all the time, seemingly personal, but I can't know unless I listen to them, which I already feel as being offensive. Is there a way to get into a conversation with famous people without being too rude?
I would set aside the 'celebrity' aspect of it. They're just people having conversations in a public place. It doesn't make them special, just popular.
Now, presuming it's an open venue and you're not blatantly eavesdropping I find it usually acceptable to politely chime in if I can offer commentary or perspective based on a specific topic in the conversation. Things like the weather or food, not so much...
For instance, recently I was in line for Pirates of the Caribbean at Magic Kingdom and the group ahead of me was debating the which Big Thunder Mountain was better, Disneyland or Magic Kingdom. Me: "Big Thunders? The version at Hong Kong beats them both because it has a reverse element." Very important, it's Big Grizzly Mountain, but the same theme. This lead to an enthusiastic conversation about the parks for the rest of the line.
You'll find the same situation, polite interjections, at all sorts of places, cruises, fan-cons, tech conferences, airplanes. I can't imagine book signings would be much different.
If they don't want you in the conversation, it will be immediately obvious.
Much here depends on the context. Some examples:
You and several of your heroes are in the "speaker room" or the "green room" or some other place set aside for those who meet some criteria, which you do. In this context you are peers. You are welcome to listen to their conversation and to join in if it seems you have something to ask or say that would be welcome. This can also apply at a speaker dinner or other event where the organization that has brought you together has labelled you as peers, no matter what you think of the other people. (The first time I was at a dinner table with the person who invented the language I work in most, I had to act as though we were peers, challenging as that was.) You may want to listen more than you talk and enjoy the chance to learn from these people.
You and other non-select people (eg attendees of the book launch) are in a public area such as a hallway, and you see some of your heroes and some other people having a conversation. Here, you are probably ok to join in and start listening, but you can't be quite certain. Come up to the group, make eye contact with someone other than the person currently speaking, and possibly say "hi" quietly. The group will let you know with body language if you've intruded. One of them may say something like "we should continue this later" and the group may either disband or start to discuss a different topic that is ok to discuss with you. Generally, people in hallways and other public areas of events know their conversations are pretty much public and don't mind others joining. You will know when someone smiles or nods at you as you join that you are not intruding.
You are lined up for coffee or food or something and you see someone you respect and admire. You want to say something to them: go ahead! (A relevant answer from me on a sister site: https://academia.stackexchange.com/questions/28665/walk-up-and-talk-to-random-people-at-academic-conferences-is-that-appropriate/28679#28679 goes into more details. I would like to emphasize, please, have a second sentence ready.)
You see a number of your heroes standing near a stage they are about to go onto, as panelists or the like, and you are not on the panel or otherwise a peer. They are talking. Leave them alone. They may be planning the panel, or otherwise doing something important and time limited. After the panel, you can come and ask questions like anyone else.
Generally when someone less famous than me wants to join a conversation, I don't mind at all and if I can, I'll say something to welcome them into the group. I also try to encourage "pac man circles" which is when a group of people standing in a circle chatting deliberately leave a gap so others can join in. The whole point of these events is to meet each other and to exchange ideas in both directions. Famous people who don't want to do that hide in the speaker room, the green room, or their hotel room. They walk quickly from place to place without making a lot of eye contact with anyone they see. People who are standing around chatting are generally open to standing around chatting, so join in!
Replace the word "celebrity" with "stranger" and you will probably figure out the answer yourself, but...
A celebrity is someone you feel like you know, but you really do not. They are a stranger to you. You do not really know them, or know how they will react to another stranger (you) approaching them out of the blue. They may be very friendly and understanding, or they may not like it one little bit. They may also be busy, and even if they are normally nice they may end up being brusque.
If you choose to approach them and say hi it is a bit of a lottery as to how they will react. Of course, the more respectful you are of their privacy the more likely they are to respond in a nice way. The worst thing is when you really admire somebody and then they are rude to you.
I do a little bit of work in the media industry and have spent time in the 'green room' at some of the biggest comic and film conventions. I've had the opportunity to sit back and observe how "normal" certain celebrities act. As such I've been able to strike up conversations with some incredibly nice and down-to-earth celebrities, but I've also recognised a few that just would not respond. Basically, if they have a massive entourage around them and they walk through the room as if nobody else is there then they don't want to be spoken to.
At an event like you describe - a book launch - the event is about promotion, and likely everybody is there for the same reason, to promote themselves or their work. It is probably the best time to speak to a celebrity. My advice would be to make it about their current work. If they have just released a new book, say hi and talk about the new book, not any old book or something they did twenty years ago. If you have some personal connection to the subject matter then speak about that. Focus on something specific you found interesting about the event.
I have also found from experience that people going up to celebrities and asking for a selfie tend to get knocked back, as they come across as a fan. But after I had spent a bit of time talking to certain people about their work I found that if I asked "hey, can I get a quick selfie of us together?" they always agreed. Approaching people on the basis of their work and acting "normal" certainly seems to put celebrities at ease more than plain out fanboy/girling.