The action of theatres during the week has been rather irregular. Among other changes an important modification has been made in the cast of Mr. Wills's play, "Mary Queen of Scots," at the Princess's - Mr. Charles Harcourt having surrendered the part of Chastelard into the hands of a young actor named Mr. Forbes Robertson, who appeared in it for the first time on Thursday week. He made a very favourable impression, and indeed showed many qualifications for the task. His elocution is very satisfactory; besides, he displayed much emotional power, and imparted pathos to the delivery of many of the speeches. His style and appearance are remarkably self-possessed, and he seizes upon special points with a readiness which testifies at once to his sensibility and judgment. The acting of the entire play is, as we have already said, greatly improved. Much controversy has arisen as to the manner in which the author has drawn the character of John Knox. For the rigid historical outline he has substituted a more flowing and varying form of individuality, admitting natural emotion to mitigate the severity of the general ecclesiastical character. For a moment the great preacher and reformer yields to the fascinations of the beautiful queen; and it requires all the force of his piety to rectify his position. Many critics have thought this treatment wrong, because it subjects a religious hero to laughter. Certainly, this appeared to be wrong upon the first night; but on Thursday, so hearty was the response on the part of the audience, that it was impossible not to feel that the judgment of the dramatist was justified by the result. The scene is evidently not intended to be a serious one, but to serve as a comic relief to the monotoy of the prevailing sentiment. Mr. Rousby has, at last, seized on the idea; and he and his wife carry it through with great spirit. The house was full, and the applause vehement.