When the disciples return, John says they marvel that Jesus is speaking with a woman. The woman then leaves her water jar and goes into the town to tell others about Jesus.
The scene crosses more than one social boundary at once: the conversation is public, extended, male-female, and Jewish-Samaritan.
Men and women certainly interacted in first-century Jewish society, so the disciples' reaction should not be inflated into a universal rule forbidding conversation between the sexes. Nevertheless, extended public conversation between an unrelated male religious teacher and a woman could attract notice, especially when the woman also belonged to a religiously estranged Samaritan community.
The water jar was the practical reason for her visit; leaving it behind highlights the abrupt change in her priorities.
The social context helps explain why John explicitly records the disciples' amazement. Jesus' conduct does not fit their immediate expectations.
The woman then becomes the person who summons her townspeople to investigate Jesus. The one initially approached across social boundaries becomes an active witness within her own community.