Designing exhibits that catch and hold people's interest -- particularly easily distracted children -- is a key challenge for science centres. Here we apply situational interest (SI), a model from educational psychology and STEM education, to understand characteristics exhibit designers use to trigger and/or maintain interest, evoke emotions and foster relevance. Australian designers were surveyed (n = 18) and interviewed (n = 5). Analysis showed although designers did not consciously apply SI, they consistently used fundamental aspects of the model, such as selecting distinct design characteristics to trigger interest, others to maintain it, and considering how characteristics affected emotions and relevance. This demonstrates SI is applicable to exhibit design. To trigger interest, designers highlighted characteristics including "accessibility," "hands-on experience," "user-centredness," and "sensory stimulation," and to maintain interest open-endedness. "Social interaction" -- especially having multiple simultaneous exhibit users -- was the only design characteristic identified as key for both triggering and maintaining interest. Designers associated specific emotions with triggering and/or maintaining interest, including negative or bi-valanced emotions. Characteristics to increase relevance included "familiar objects," "prior experience"/"prior knowledge," "novelty and familiarity" and "social and cultural context." This study fuses psychological theory on interest with the lived-experience of exhibit designers, providing insights useful for designers, interpretive staff and researchers.