This paper aims to examine the stylistic features in the thought asides in Eugene O’Neill’s Strange Interlude (1928) by means of lexical and syntactic information obtained from the corpus of the play. No other play created during the playwright’s experimental years (1920–1934) achieved greater popular success than Strange Interlude, in spite of the potential risks of leaving a discursive impression on the characters’ speeches, and of the nearly six-hour performance length overwhelming both actors and audience. Although a number of studies have been put forward on the use of thought asides, it cannot be said that this subject has been fully researched because there has been no corpus stylistic analysis of the thought asides in the play. Thus it is worthwhile to conduct such an analysis using lexical and syntactic information gained from the corpus. In order to explore these issues, this paper deals with the following four points: in Section 1, giving a brief overview concerning the corpus data of Strange Interlude; in Section 2, conducting a lexical analysis of Charles Marsden’s thought asides so as to examine his thought presentation, as such an analysis and examination can indicate any connection between features in his vocabulary and his characterization; in Section 3, dealing with the frequency of the suggestive phrase “got to” in Mrs. Evans’s thought asides as a form of reflection of her anxiety and sense of duty; and finally, in Section 4, examining one of the most characteristic differences in style between two types of thought asides—one developing a character’s thoughts in an objective manner, and the other revealing their emotional feelings. Through investigation via corpus stylistic analyses, this paper attempts to shed light on the various stylistic features in the thought asides in Strange Interlude.