As a presenter, particularly if you have been given preparation time, know your subject. If you don’t know your subject, then it will become patently clear to the audience that your knowledge is stretched. Restlessness, figitiness , looking uncomfortable, visible sweating and other visible manifestations will become giveaways. Eye blinking and throat clearing might become part of the reaction all too visible to the listening group.
‘Subject stretch’ will bring out uncharacteristic ‘ahs’, ‘ums’, ‘you knows’ and so on in an altogether uncharacteristic manner.
There is a lot to be said for being prepared.