Abigail's attraction to John Proctor can be seen when she admits it and attempts to cling to him in Act 1.
In Act 1, John and Abigail find themselves alone together. John is definitely uncomfortable with the situation, but Abigail is wonderfully excited to be alone with John again. John attempts to leave, but Abigail rushes to block his path. She then begs for John to say a "soft" word to her.
He takes a step to go, and she springs into his path.
Abigail: Give me a word, John. A soft word. Her concentrated desire destroys his smile.
Essentially, Abigail is hinting that she wants John to "whisper sweet nothings" to her, as John likely has before. Stronger evidence in support of her attraction to John is in the stage direction about "her concentrated desire." Abigail wants more than mere small talk with John. She is attracted to John, and she's not afraid to remind John of the passionate times they have shared, saying,
I know how you clutched my back behind your house and sweated like a stallion whenever I come near!
Abigail is also not afraid to say she wants those illicit rendezvous again.
Abigail, grasping his hand before he can release her: John — I am waitin' for you every night.