Several years ago I was doing a play called TALLULAH AND TENNESSEE with a "name" actor. A majority of the scenes were just two character scenes between myself and this "name" actor. Unfortunately, the "name" actor never learned his lines. NEVER! He finally resorted to putting pieces of paper with his lines written on them all around the set. However, when he would forget his lines he would very often go to the wrong place on then set and start saying lines from another scene or another act altogether. For example, if the correct lines were on the coffee table, he might forget that and go to the desk and say lines from the wrong scene. Sometimes when he went up, he would just start singing! I ended up learning all of his lines as well as mine so I could keep the dialogue on track by prompting him or moving him to the place on stage where his lines were written. This went on for the entire run. The play was never performed as written, ever! Very challenging and quite scary. Talk about having to think fast!