直(ただ)ちにAt once, Immediately, As soon as

Both 直ちに and たちまち mean 'immediately', but differ in tone and context. 直ちに is formal and emphasizes urgency, often used in official or written language, e.g. '直ちに避難してください' ('Please evacuate immediately'). たちまち suggests a spontaneous change out of the speaker's control, e.g. '空がたちまち暗くなった' ('The sky suddenly turned dark').

Both express immediacy between actions, but with different focus. 直ちに stresses promptness, often used for direct commands or instructions, e.g. '直ちに医者を呼んでください' ('Call a doctor immediately'). か〜ないかのうちに highlights how closely two actions occur in time, e.g. 'ドアを開けるか開けないかのうちに猫が飛び出した' ('No sooner had I opened the door than the cat jumped out').

Both involve rapid transitions, but differ in usage. 直ちに is formal and emphasizes urgency, often used in official or written language, e.g. '直ちに避難してください' ('Please evacuate immediately'). かと思ったら・かと思うと is more colloquial and marks an abrupt or surprising change just after a perceived moment, e.g. '転んだかと思ったらすぐ立ち上がりました' ('Just when I thought he'd fallen, he got right back up').

直ちに and たとたんに both indicate swift timing, but differ in tone and structure. 直ちに is formal and emphasizes urgency, e.g. '直ちに避難してください' ('Please evacuate immediately'). たとたんに conveys something happening instantly after another event, often with surprise, e.g. '顔を見たとたんに泣き出した' ('She started crying the moment she saw his face').

Both 直ちに and 次第 can imply prompt timing, but differ in grammatical usage and nuance. 直ちに is used before phrases and emphasizes urgency, e.g. '直ちに避難してください' ('Please evacuate immediately'). 次第 attaches to verb stems to suggest one action happening immediately after another, e.g. '準備ができ次第、出発します' ('We'll leave as soon as we're ready').