I don't like improvising as an actor. I never have. I'm not sure why. Maybe it's just that I prefer the comfort of knowing there's a script and knowing where the scene is going. Maybe it's lazy, because if nobody else is doing anything too strange, I know how to react to it. Maybe it's fear - since I so often say things the wrong way or baffle people with my words (not by being clever, mind you, just by choosing the strangest, most awkward possible phrasing) I worry that I will derail the whole scene for everyone else and look like an idiot on stage. Or maybe it's a learned response, over years of half-written church skits. When people at a small-to-medium-sized church hear that you like to act, every now and then they'll grab you to be in a skit, which usually involves a vague briefing five minutes before the service telling you to "just stand over there with those people, say some mean things, then fall over when I throw the box." It's the second-most awkward possible form of acting I've ever done, the first being job interviews.
At the same time as feeling awkward and inadequate when improvising, I recognise that it's a pretty big part of the job. If you can't get into the head of a character and at least work close to the script if not completely off the cuff, then you're not much good as an actor. Missing a line is definitely going to derail the whole scene, throw off the other actors and make me look like an idiot.
And, of course, like so many others of my personal flaws, I seem to have only enough self-awareness to recognise it, not to know how to fix it.
Mokalus of Borg
PS - If I figure that part out, I'll let you know.
PPS - Exposure therapy would probably help.