Yes, Grace, interfacing is usually required. It helps to give body, support and shape to the neck facings, front facings, cuffs, collars, etc. Otherwise, those areas would be limp and lifeless. The easiest way to go is to use fusible interfacing. After cutting out the interfacing piece, I always trim 5/8" off each edge to decrease the amount of bulk. With fusibles, you don't need to capture the interfacing within the seam to make it stay in place, so you can trim it like that. You should also trim diagonally across the interfacing collar points and corners of the cuffs to reduce bulk in those areas. I also trim sew-in interfacings, but then I tack them in place. The interfacing also gives backing to design elements like buttonholes and buttons, that, without the interfacing behind, would just sorta flop around. Some very casual tops may have you just put a bias piece around the neck opening instead of an interfaced facing, but those are usually patterns for beginners to get them started.

A good rule of thumb is to buy interfacing that is slightly lighter in weight than the fabric you are using it on.

Happy sewing!
Mary