To meet the requirements of students' cooperative learning, our quasi-experimental study addresses how preservice teachers (PSTs), as a component of their procedural pedagogical content knowledge (PCK), might develop more adequate tutor-student interactions during students' experimentation. Applying the assignment-assistance tutoring model, we combined biology PSTs' education with our high school students' outreach program, Genetic Fingerprinting. We developed a role-play-based tutor training in a 2-step (role play and group discussion) and in a 3-step variant (role play, group discussion, and an additional assignment exercise). We audiotaped and transcribed all PST-tutored experimental phases of one control group (without training) and our two training groups. We categorized 2,865 tutor-student interactions content-analytically regarding adequacy in potentially supporting students' promotive interaction as a core condition of cooperative learning. We examined the PST interaction patterns identified. Our tutor-trained PSTs, especially those who received 3-step training, showed less inadequate tutor-student interactions compared with control group PSTs. Cluster-analytically extracted intervening interaction types were found only in the control group and the rule-complying type dominated the 3-step group. We discuss our training as an effective approach to develop tutor-student interactions as a component of procedural PCK that is part of PSTs' initial professional development.