Like some other's have said. Write everything down and keep a copy. Use this as the basis for the police report, and insurance and do all ASAP if not already done. Encourage the witness to also write things down.
Keep your copy, it may go to court some time from not you will be asked for details... it is best if they have not faded or worse yet changed (from memory fading) with time.
No detail is too small. Colour, make, model or vehicle. Damage to their vehicle. Description of driver and witness, whatever.