When an ISTJ has a crush on someone, they may exhibit a combination of subtle and direct behaviors. Here is a detailed description of how an ISTJ may act around their crush:

1. Reserved and Observant: ISTJs are typically reserved individuals who prefer to observe and analyze situations before taking action. When they have a crush, they may become even more observant, paying close attention to their crush's behavior, interests, and preferences.

2. Polite and Respectful: ISTJs value manners and respect, so they will likely be polite and respectful towards their crush. They may use formal language, hold doors open, or offer small gestures of kindness to show their interest.

3. Shy and Nervous: ISTJs can be shy and introverted, especially when it comes to romantic feelings. Around their crush, they may become more reserved, finding it difficult to express their emotions openly. They may feel nervous or anxious, leading to a slight awkwardness in their interactions.

4. Thoughtful and Considerate: ISTJs are known for their thoughtfulness and consideration towards others. When they have a crush, they may go out of their way to do things that make their crush happy or comfortable. They may remember small details about their crush's preferences and surprise them with thoughtful gestures or gifts.

5. Reliable and Dependable: ISTJs are highly reliable individuals who value commitment and loyalty. When they have a crush, they may make an effort to be there for their crush whenever needed. They will likely keep their promises, be punctual, and offer support whenever their crush requires it.

6. Reserved Displays of Affection: ISTJs may struggle with openly expressing their feelings, especially in the early stages of a crush. They may show their affection through subtle actions, such as offering a helping hand, listening attentively, or providing practical assistance. They may also give compliments, but they tend to be sincere and specific rather than overly romantic.

7. Analytical and Logical: ISTJs have a logical and analytical approach to life. When they have a crush, they may analyze their feelings and the potential outcomes of pursuing a romantic relationship. They may take their time to assess compatibility and ensure that their crush aligns with their values and long-term goals.

8. Reserved Flirting Style: ISTJs may not be the most overt or playful flirts. Instead, they may engage in reserved flirting, such as engaging in intellectual conversations, sharing their knowledge on a subject of interest, or offering practical advice. They may also show their interest by being a good listener and remembering important details from previous conversations.

9. Slow to Open Up: ISTJs tend to take their time to open up emotionally. They may be cautious about revealing their true feelings until they feel secure and confident in the relationship. This can sometimes make it challenging for their crush to gauge their level of interest initially.

10. Loyal and Committed: Once an ISTJ has established a connection with their crush and decides to pursue a relationship, they are likely to be extremely loyal and committed. They will invest time and effort into building a strong foundation and maintaining a stable and reliable partnership.

It's important to note that individuals may vary in their behavior, and not all ISTJs will exhibit the exact same actions when they have a crush. However, these characteristics generally align with the typical behavior of an ISTJ in a romantic context.