The art of improvisation is at its most popular since the heyday of Whose Line Is It Anyway? X-Files star Dean Haglund has just had an on-the-hoof hit at the Trafalgar Studios, Paul Merton has just launched a prime-time ad-libbed ITV show, Thank God You're Here, and ImproFest 2008 is an ambitious celebration of the genre.

On Friday a 50-hour "Improvathon" kicks off, sleeping bags optional.

Until then evenings offer new nightly musicals. The opening Showstopper! might not even have had the depth of a Lloyd Webber production but when it was not too chaotic it had its comic moments.

Bearded director Dylan Emery, a dead ringer for the Elizabethan Blackadder, started proceedings by taking audience suggestions for the unwritten production. His fearless cast of eight plus pianist then had to deliver an entire show combining funk, opera, Gilbert & Sullivan and Bob Fosse and opening in a funeral parlour.

The result - a Sondheim-ish hotch-potch of murder, intrigue and jazz hands in wartime London - was inconsistent, though it would have been a lot less inconsistent if the cast's talents had been more reliable. Singing ability was decidedly varied, from the formidable to the positively feeble.

Two performers did impress. Adam Meggido exuded a nice mix of menace and vulnerability as befuddled arms dealer Archie McSlag, while Ruth Bratt delivered two striking set-pieces as his streetwalking amour, Misty. The difficulty was resolving a plot that became more tangled with each song.

Undoubtedly an intriguing live experience, but you can see why Whose Line Is It Anyway? was pre-recorded and edited.