If someone has a mobility impairment do not assume this means they use a wheelchair. They may use crutches or a stick, or have a condition such as arthritis which makes fine control of movement difficult. They may use a wheelchair on some occasions and not on others.
Never move someone's crutches, walker, cane, or other mobility aid without permission.
A wheelchair is part of the body space of the person who uses it. Do not lean on it. Do not crouch down when speaking to a wheelchair user. Attempt to put yourself at the same level by sitting on a chair.
When meeting people who are wheelchair users, or people who have difficulty walking, remember that accessible venues should be chosen eg on the ground floor with level access, or served by an accessible lift.