The verb of perception (see, watch, notice, hear, listen, smell, feel) are followed by bare infinitive or by -ing form (present participle).
- If we want to say that we heard or saw the whole action from beginning to end, we usually use bare infinitive
- If we want to say that only saw or heard part of the action, we use -ing form
I saw him sign the cheque.
I saw the consultant waiting in reception.
(I saw consultant. He was waiting in reception.)