When to promp?

To prompt, or not to prompt, that is the question... Having worked extensively with both "no prompt" and "prompt available" my gut reaction was that they should only swing into action if that horrible, deathly silence falls. I have seen, and taken part in one, amazing "saves". Most notably the occasion that someone did not enter, we looked at each other, and launched into ad lib that would have had the author spinning in his grave. We kept to script as best we could, but there were some sequences our characters would not, could not, know about. Missing person entered, was given a "you've missed the fun, we've just found out that X happened" speech and we were back on track. I never want to go through that again. Talk about sweating. I was also doing a one woman piece and totally, completely dried. I was massively stressed and physically ill prior to performance, in hindsight I shouldn't have gone on. Thank god for the SM who prompted, when asked. Another scenario, similar to Gordon's question. Two characters went off script, still not sure why, rambled on for a bit but were flowing and in character. Prompt was going crazy in the wings trying to work out what to do if called. Third character entered early and put them out of their misery and back on track. Reflecting on all of the above, I come down on the side of "don't interrupt unless asked", even if actors are way off beam. Most casts will pull together and rescue each other. Where a prompt is not provided during the season it's my experience that this is particularly the case. Some actors just have the gift of being able to ad lib and thus, can help out those in trouble. I still feel sorry for the author's mangled script, particularly if I am the culprit! "Papa, where is Mama? They tell me she has gone away, where is she?"