Telling the reader a character is attracted to someone is simple. And rather flat and boring. But showing several small attraction signs can give the story a strong punch that will glue the readers to the pages and make them relive their own troubles and beautiful moments.
Clumsiness and nervousness
Characters that do not typically bump into things or have troubles with fluent speech can find out their own body betrays them in the presence of their interest. They can stumble, drop a pen, find it hard to push an elevator button… The level of attraction, especially when it is just blooming and there is no deeper relationship between the characters can manifest in the degree of this unwanted clumsiness. A character ear-deep in trouble (love) can miss a stair and injure themselves while only mildly attracted character can simply hit the stair with the tip of their boot and go on.
The same goes for talking. Stuttering, failing to find the right words, or even inability to speak to the so interesting love interest can be great signs for the readers (and the other characters). The tone of voice can change as well.
Being attracted to someone can cause unintentional gestures or actions. Nervous characters can play with something in their hands, be jittery, reorder things in front of them, keep opening and closing a book…
Involuntary body response
Being close to an attractive person can result in many uncomfortable bodily reactions. From sweating, having goosebumps, or blushing to changes in the private parts. Attraction can cause a rise of body temperature, quicken the heartbeat, change breathing patterns…
The intensity of these changes can vary, depending on the time and physical closeness. A character can hear an attractive voice and experience faint tingling around the ears and stomach. The situation can develop as the attractive character walks in, causing a warm feeling in the other one. The scene can gradually continue with physical contact or a dialogue that will push the attraction signs forward.
Controlled and uncontrolled behavior
The upper mentioned signs are great for characters who are a bit of a victim of their own feelings. They can experience their first falling in love. Or not feel comfortable with the situation for any other reason. They find it hard to react and the possible relationship development can take a long time.
But you can and should write also characters who know what they want and consciously woo the attractive character. They can suffer from mild uncomfortable signs as above but they usually take an active role and use their body language and actions to pursue their goal.
They can consciously change their voice, adjust their appearance, willingly lean closer to the object of their attraction, use touches and gestures to create the desired atmosphere… Such characters usually do not hesitate to engage in frequent conversations, make plans for both, ask bold questions, or rearrange the surroundings to achieve an easier interaction. You can make the wooing persons stand up and wave in a room full of people. Or make room for sitting really close in an empty environment. Put aside objects to have a better view, use every opportunity to touch the other one…
The lured character can respond in various ways, showing the same active and decisive behavior. Or you can, again, use some of the less comfortable signs to show what is on the other person’s mind.