Fostering not only means caring for the cats physical needs. It also prepares the cat for spending the rest of its life with humans.
That means that foster parents are a crucial step in the socialisation process of the foster animal.
Please be aware that every cat is different and will settle into a foster home differently. Some need more time while others are comfortable from day one.
Some foster cats require more intensive care when they show timidness, aggressive or destructive behaviour, activity at night or litter box issues.
As a foster parent, you must be willing to work on such issues with the foster cat(s).