Research on decision-making skills of teachers has indicated the importance of experience. Also, occupational culture has been found significantly related to teachers' thinking and behavior in the class. This study investigates in what aspects experienced teachers (cooperating teachers) and less experienced teachers) (student teachers) differ when making instant decisions on diverting from their lesson plans as they teach, and what cultural/institutional influences can be related to these differences. The data for this qualitative study came from four cooperating teachers and seven student teachers through several research instruments: video-taping the participant teachers' lessons, the interviews held at post-teaching meetings, the written retrospectives, and the questionnaire. The findings indicate that while 'timing' and 'classroom management' were major motives for student teachers to make changes in their plans, cooperating teachers were more concerned about discipline problems. Both groups of teachers were equally affected by 'motivation', 'physical conditions' and 'language skills' in making instant decisions. The paper ends with the implications of the study for teacher training and education.