Many of the activities and cognitive processes that physicists use while solving problems are "invisible" to students, which can hinder their acquisition of important expert-like skills. Whereas the detailed calculations performed by researchers are often published in journals and textbooks, other activities such as those undertaken while planning how to approach a problem are rarely discussed in published research. Hence, these activities are especially hidden from students. To better understand how physicists solve problems in their professional research, we leveraged the framework of cognitive task analysis to conduct semi-structured interviews with theoretical physicists (N = 11). Here we elucidate the role of planning and preliminary analysis in theorists' work. Theorists described using a variety of activities in order to decide if their project was doable while also generating possible solution paths. These actions included doing preliminary calculations, reflecting on previous knowledge, gaining intuition and understanding by studying prior work, and reproducing previous results. We found that theorists typically did not pursue projects unless they had a clear idea of what the outcome of their project would be, or at least knew that they would be able to make progress on the problem. Thus, this preliminary design and analysis phase was highly important for theorists despite being largely hidden from students. We conclude by suggesting potential ways to incorporate our findings into the classroom to give students more numerous opportunities to engage in these expert-like practices.