There is a growing need for more effective interdisciplinary science instruction across undergraduate degree programs. In addition to supporting students' connections between disciplinary concepts, interdisciplinary learning can develop students' critical thinking skills and allow them to evaluate scientific investigations and claims between diverse topics. Physics Education Research literature has particularly focused on introductory physics courses for life sciences students, in part because students majoring in life sciences represent one of the largest demographics enrolled in physics courses. This literature has primarily focused on students' development of conceptual understanding, modeling skills, and perspectives of the two fields. In this study, we explored how biology and physics undergraduates approach and perceive critical thinking between the two disciplines. We conducted structured think-aloud interviews with biology and physics students, asking students to first complete portions of established biology and physics critical thinking assessments and then respond to several follow-up questions about critical thinking more generally. Using thematic analysis to inductively code interview responses into emergent themes, we found that most students, regardless of major, described different approaches to evaluating biology and physics experiments. However, physics students provided similar definitions of critical thinking in the two disciplines, while biology students provided similar and different definitions in almost equal numbers. The exception was related to the use of quantitative methods solely being associated with critical thinking in physics, despite both critical thinking assessments involving quantitative data analysis. When looking across constructs, we saw no clear trends or relationships between individual students' responses to each of the interview questions. We also explored students' broader perspectives on the two fields and found that physics students assume that physics is needed to understand biology but not vice versa, which did not align with their perspectives on critical thinking between disciplines. We use this complexity to motivate future work to understand the impact of biology and physics instruction, as well as other STEM disciplines, on developing students' critical thinking skills and perceptions.