What questions should I ask a client about a slip and fall incident?

Everything. For example:

1) Get as thorough a description of the incident as possible. Be sure to focus in on: (a) lighting conditions; (b) physical conditions at the site; (c) weather conditions (if applicable); (d) anything slippery, uneven, etc.; (e) what was the client doing at the time; (f) who else was there; (g) did anyone else bump, push, etc. the client; (h) what was client wearing; (i) what was client carrying; (j) if there was anything wet, uneven, etc., did client notice it ahead of time or have any warning otherwise of it; (k) the date and time of the incident, as well as the timeline

2) Clients physical shape/condition: (a) anything that might contribute to the fall (bad knee or ankle, overweight, etc.); (b) anything that would exacerabate, or be exacerbated by, the fall (e.g. a pre-existing back problem)

3) Medical care rendered--complete detail, including who (dr. or therapist), where (hospital or office), when (dates and times of visits), what (kind of treatments), and why (any explanation); limitations on client's activities and prognosis

4) Client's job, income, etc. Whether they have been unable to do their job since the injury. Client's hobbies or interests; anything they have had to give up owing to the injury.

5) Who have they told what about the incident; any correspondence, including email and posts on social network sites, they have had about the incident