ところに / ところへ

1. 1Just as someone was about to do something, another event occurred and interrupted it.Ex. 1
2. 2While someone was in the middle of doing something, another event happened and affected the situation.Ex. 2
3. 3Right after someone had done something, another event occurred.

Common in narration and in everyday accounts of events. The main clause describes an event that arrives at or intrudes on the situation, such as a visitor, a phone call or the weather, often with verbs like 来る, 現れる or かかってくる. It can also be someone's own action that intrudes, as in 寝ているところに電話をかけてしまった. ところへ slightly highlights movement or arrival toward the scene, but the two forms are largely interchangeable.

• ところをN2「ところを」 treats the scene as the object of the following action, such as being seen, caught, or stopped, and is often used in polite set phrases like 「お忙しいところを」; 「ところに」 only means something else happens to occur at that moment.
• 最中にN3「最中に」 only attaches to a noun plus の or a verb in the ている form, emphasizing being interrupted while an activity is still underway, and can also act as a predicate as in 「最中だ」; it has no use for being about to do or having just done something, and does not attach to adjectives. When expressing an ongoing action, the two are often interchangeable.
• 途中に / 途中でN3「途中に / 途中で」 focuses on the middle stage of a process or journey and does not stress coincidence or a feeling of being interrupted. It can also mean a point along the way, as in 「駅へ行く途中に銀行がある」, or giving up halfway, as in 「途中でやめる」, and the preceding verb does not take the た form.
• かと思ったら / かと思うとN2「かと思ったら」 indicates that right after the first item occurs, a contrasting change immediately follows, and can also mean 'I thought... but...'; 「ところに」 means that while one is at a certain moment, an outside event happens to intervene, so the two are generally not interchangeable.
• たとたんN3「たとたん」 only attaches to a verb in the た form and stresses that the moment the first action happens, the second immediately follows; 「ところに」 has flexible attachments and stresses that an outside event arrives while one is at a certain moment, without highlighting immediate succession.
• 間にN4「間に」 sets a time span with a beginning and end, and the main clause describes something occurring within it, with an objective tone and no sense of 'of all times'; it can also express intentionally completing something before the period ends, so the following clause can take volition or wishes, which 「ところに」 cannot.

ところを is used when the main verb acts directly on the person in that situation (見られる, 助ける), whereas 最中に and 途中で focus only on the midst of an activity or process, and 間に simply means within a time span. たとたん and かと思ったら stress a sudden change immediately after an action, while ところに covers before, during or after and highlights another event intruding on the situation.