There are various synonyms that can be used instead of "got serious" to convey the same meaning in a sentence. Some of these synonyms include "became grave," "turned somber," "got earnest," "became solemn," "got focused," "became intense," "took seriously," "became sober," "got down to business," and "became attentive." These words can be used to show a change in attitude or behavior towards a particular situation or activity. By using different synonyms for "got serious," writers can add flavor and variation to their writing while still conveying the intended meaning effectively.