In the third film (Harry Potter and the Prisoner of Azkaban), when the Dementor enters the train carriage, Prof. Lupin casts the silent spell which is obviously Expecto Patronum. However, he only manages to conjure the whispy shield, rather than a fully formed (corporeal) patronus. Even though it is shown later in the film that he's actually extremely skilled with the spell.
Did he only use a weak memory to not get a patronus running around the carriage? or was it just because he had woke up and didn't think fast enough?