This paper explores the influence of first language and level of development on the use of conventional expressions in the realization of three speech acts, namely, expressions of gratitude, apologies, and refusals. An experimental approach reproduced the conditions for the use of conventional expressions employing a computer-delivered aural-oral discourse completion task (DCT) with timed presentation of scenarios and a recorded interlocutor to simulate turn taking in scenarios that promoted high use of conventional expressions by native speakers. Learners from four levels of proficiency representing four first language groups (Arabic, Chinese, Japanese, and Korean; n=108) and native speaker peers and teachers (n=49) participated in the study. Use of conventional expressions was in part mediated by first language and instructional level. The comparison of multiple L1s showed that learners of various L1s often shared production strategies. The comparison of different levels of instruction showed that learners increased their use of conventional expressions at higher levels, requiring both linguistic and sociopragmatic competence.