Some people come straight to improv without any traditional theatrical training and some improv schools don’t spend a lot of time working on basic stagecraft. I have only the slightest of scripted theatre backgrounds but I did learn early on that in scripted theatre, fight scenes and love scenes were best approached with respect for personal boundaries, safety, caution and above all that it was important to take time to establish a level of comfort and trust between performers. If you’re doing a long-form improv show that’s had a rehearsal process there is more time to delve into these issues but in short-form it often comes down to evaluating all of these things in the moment. If an improviser doesn’t immediately know whether physical contact with their scene partner will be outside of their comfort zone, the impulse is often abandoned. Generally speaking I think we sometimes favor caution out of respect which is not a bad thing. The more people work together the stronger the communication and then you don’t have to second-guess those moments.
Offstage, I’m a hugger but I try to be sensitive to people who may not be so keen on hugging. The same is true onstage. If I’m in a scene that turns romantic and I’m with someone I haven’t improvised with very much, I tend to hang back and let them initiate physical contact if they want to. Once in a great while I’ve been known to plant one on somebody but usually it’s only if I have a sense, based on past experience, that they’ll be cool with it and then only if it’s appropriate for that particular scene.
Occasionally there is a concern that if an improviser is in a romantic scene involving a kiss or other physical contact onstage it may not sit well with a significant other. As someone’s wife I can understand that even though I don’t feel the same way (but then again we are both improvisers). As a performer, I have been kissed onstage by plenty of people that I’m not attracted to and to me it’s no different than say, shaking someone’s hand onstage or playing their arms. It’s acting which is not the same as dating. But I respect that for someone in the audience (especially if they’re not a performer) it might feel a little weird to them to watch their girlfriend, husband, etc… kiss someone else onstage. For performers who know that their significant other isn’t cool with it (whether they’re in the audience that night or not) sometimes this causes them to refrain from physical contact onstage and that’s their choice which is valid.
Also a thought about stage-combat. Never hit, slap, kick, throttle, pull the hair of or in anyway jump on another performer in an improvised scene onstage without some prior communication ever. Ever ever ever. If you see an actual open-handed slap (for example) onstage in a scripted show, unless it was an accident, it is likely to be something the performers worked out in advance so that no one was hurt because it was choreographed in a very specific way. If someone hurts you onstage, talk to them. If it’s an ongoing issue with a specific person then the leader/Artistic Director of your group should be made aware of it and handle it. Even if you are a very physical performer it is important to be careful of moving erratically and avoid putting yourself in a position where the audience or your fellow performers might worry that you could do harm to yourself or others. Above all - being out of control physically & straight-up physical violence are never OK.
What I mean by a “kiss”: I mean a “stage kiss” which is lips closed, no tongue, no slobbering and no coping-a-feel. If you’ve been improvising with someone for years and you are both totally cool with being a little over the top or playing it up for the sake of comedy I say go for it. It can also be equally fun/funny to do some cheesy-makeout-mime where you sort of simulate making out but have some obvious physical distance (as opposed to dry-humping). But again all of those things depend on the level of comfort between performers.
I personally love it when I see improvisers who are comfortable with each other get physical onstage. I mean this both romantically and otherwise. I’ve had fellow performers stick a hand or head up my shirt to simulate childbirth or alien impregnation (you know, like you do). This is the sort of thing you can only do when you’re REALLY COMFORTABLE WITH SOMEONE so for the love of all that’s holy please don’t go out and try that tonight in a show for the first time. If this subject is something that you’ve thought about in regard to your own group I think it’s nice to bring in up in a workshop setting. I’ve done long-form shows in which, given the style of show, it was likely that romantic scenes were going to happen and in rehearsal we’ve had a show of hands to indicate who was cool with kissing and who was not and those boundaries were respected. It’s just as valid to say “I’m not really comfortable being kissed onstage” as it is to say “Go for it! In fact, breath mints all around!” As with anything else in improv, I think good communication always helps.