Evaluating actors during a casting call is rarely an exact science. Because statements like "he is the best" or "she is the only one" express a subjective evaluation rather than an objective reality, French frequently uses the subjunctive mood in the clauses that follow them.
Superlatives
When you use a superlative expression—such as le meilleur (the best), le plus (the most), or le moins (the least)—followed by a relative pronoun like qui (who/which) or que (whom/that), the verb in the following clause takes the subjunctive.
| He is the best actor that we have at this casting call! | C'est le meilleur comédien que nous ayons à ce casting ! |
| It is the most remarkable role that she is performing for us. | C'est le rôle le plus remarquable qu'elle interprète pour nous. |
Notice how the subjunctive highlights that this is the best or most remarkable in your personal experience or opinion, rather than an undeniable, universal fact.
| This casting call is the most difficult that I am doing with them. | Ce casting est le plus difficile que je fasse avec eux. |
Absolutes
The exact same rule applies to absolute adjectives like seul (only) and unique (unique/sole). Because they express a sweeping, absolute limitation based on the speaker's perspective, they also trigger the subjunctive.
| It is the only audition that is really convincing. | C'est la seule audition qui soit vraiment convaincante. |
| He is the only capable actor that we are looking for. | C'est le seul comédien capable que nous recherchions. |
As always in French, the relative pronoun qui acts as the subject of the following verb, while que (or qu' before a vowel) acts as the direct object.
| She is the unique actress who is remarkable. | C'est l'unique actrice qui soit remarquable. |