Simplify the following text for a Grade 8 native English-speaking person.
Original: Making the film had proved to be an arduous experience for the actors ; in addition to Monroe 's distress , Montgomery Clift had frequently been unable to perform due to illness , and by the final day of shooting , Thelma Ritter was in hospital suffering from exhaustion .
Simplified: In addition to Monroe 's problems , Montgomery Clift was often unable to perform due to illness . By the final day of shooting , Thelma Ritter was in hospital because of exhaustion .