The problem with looking for a generic cinema buddy
A broad request such as 'who likes films?' creates another coordination problem. You still have to agree the title, location and time. Often the plan dies in that negotiation.
Costar treats the exact screening as the filter. Both people independently choose the same performance first. That means the important logistical agreement has already happened before either person sees the other.
Why London makes this useful
Central London offers a dense mix of multiplexes, boutique cinemas and premium formats. The problem is not a lack of films. It is getting two adult calendars to overlap at the same moment.
By concentrating Costar around a defined group of cinemas with verified screening feeds, the product can focus demand instead of scattering it across hundreds of venues.
Choose the screening you would attend anyway
Do not pick a film because you think it will be easier to match. Pick the film you genuinely want, at a time you can actually make.
That protects the quality of the introduction. If a match happens, the overlap means something. If it does not, you have not designed your evening around a stranger.
What to do when there is an overlap
Read the profile as a yes-or-no decision for this particular screening rather than a biography to analyse. If both people say yes, follow the payment and confirmation steps and meet at the cinema.
Keep the plan simple. A clear meeting point, independent travel and enough time before the film are more useful than a long pre-meeting conversation.