Improvisation is so vitally important, I’d go so far as to say:
If you can’t improvise confidently and fluently to a familiar piece of music, you’re not ready to take solo bookings.
I say that for two reasons.
One is a a matter of principle. Belly dance (raqs sharqi) and flamenco are solo improvisational dances. If you’re still totally reliant on choreography, you haven’t mastered one of the most fundamental aspects of your art. If you haven’t mastered your art, you’re not a professional, and shouldn’t be representing yourself as such to the public. And I say that as someone who still struggles with improvisation. Personally, I find it the hardest thing of all.
I have been reminded that many great Egyptian dancers routinely choreographed their whole show, but that’s not my point. You should be able to improvise, even if you don’t use it very often.
But the other, more important reason is entirely practical: there’s a very high probability you’ll have to improvise part of your performance at a paid gig.

Performing in a private home or function center is a totally different ballgame from dancing on a wide, empty stage. Space is always an issue! I’m always amazed how many sensible function center managers, who would set up a stage for my flamenco performances without needing to be asked, will blithely assume I don’t need much room to belly dance – “you just dance around the tables, don’t you?”
No matter how much you explain to the client beforehand, you’ll turn up at the venue to find there’s no central open space where you can give your performance.. Or if they have left a gap somewhere, it’s not nearly big enough for you to swing your Assaya stick safely – or there’s a huge pillar or buffet table between you and half the audience. For private gigs I always take a very light, soft silk veil, with no trim – because I know I’m going to be wafting it dangerously close to my audience’s faces!
That’s where improvisation comes in. If it’s too dangerous to perform your Saiidi dance with a stick, you’ll have to do it without – and that means changing the choreography on the fly. If your veil is going to land in someone’s soup, you’ll have to cut out the flicks and do more moves close to your body. And if there’s no central stage, you’ll look ridiculous if you do your whole performance wedged between two tables – you need to move around, which means turning all your static moves into travelling steps.
You may cope with all those things without improvisational skills, but your performance will suffer: if it doesn’t show in the quality of your movement, it will likely show in your face.
There are some good online courses to help with improvisation. Before you enrol, consider why you find improvisation difficult, because everyone is different.
Many people have found Alia Thabit’s course extremely helpful. Unfortunately, I’m not one of them. It assumes that you’re already able to get up and “just dance” to music, and it teaches you how to add discipline and focus to that, so you actually produce something that’s entertaining and cohesive, rather than just wafting around or repeating the same moves.
That’s not me. I can put a piece of music on, and I’ll sway, or I’ll jig my shoulders, but ask me to actually dance and I’ll stand there frozen, unable to think of a single move. What I needed was a course to get me over that difficulty before I could even consider something like Alia’s course. I found the Bellydance Geek’s course helpful.