こんにちは。
「アドリブ」は英語でも ad-lib（アドリブ）です。
【例】
I had to do ad-lib when I forgot my lines.
「[セリフ](https://eikaiwa.dmm.com/uknow/questions/16382/)を[忘れた](https://eikaiwa.dmm.com/uknow/questions/88707/)ときアドリブしなきゃならなかった」
The actor's ad-lib was amazing.
「俳優のアドリブが素晴らしかった」
ぜひ参考にしてください。
If you say something off the cuff, it means that you haven't thought about, or prepared for something beforehand. You could also use the word 'spontaneously' when someone does something or reacts without any preparation. If an actor forgets their words and substitutes their own, they may be said to be adlibbing - or 'winging it.'
To wing it = to do something (difficult) without or with insuffficient preparation.
「off the cuff（即席で）」は、前もって考えていなかったり、[準備](https://eikaiwa.dmm.com/uknow/questions/36245/)していなかったことを言う場合に使えます。準備なしに何かをしたり反応する場合、「spontaneously」も使うことができるでしょう。俳優が台詞を忘れて、自分自身の言葉を代用したら、「adlib（[即興](https://eikaiwa.dmm.com/uknow/questions/66324/)でやる）」や「wing it（即興でやる）」が使えます。
To wing it = 何か（困難な）ことを（十分な）準備なしで行うこと。
When an actor speaks without a script, then you can use the phrases "ad-lib", "winging it" or "improv".
Remember that "improv" means improvisation = this mean that the person is doing something they did not really prepare. Winging it also has the same meaning.
For example:
-As an actor, you have to do an improv class.
-Brad Pitt ad-libs some of his lines in his movies.
台本なしで俳優が話す時、ad-lib、winging it、またはimprovというフレーズが使えます。improvは、即興（improvisation）という意味で、準備無しで何かを行うことを言います。winging itも、同じ意味です。
例：
-As an actor, you have to do an improv class.
俳優として、即興の授業を受けなくてはなりません。
-Brad Pitt ad-libs some of his lines in his movies.
ブラッド・ピットは、映画でアドリブを言います。
When an actor doesn't rely on a script and says things the way they want to it is called ad-libbing, improvising, or making it up as they go along.
"He began reading the script at first but then he began ad-libbing."
"When she is on stage she always improvises."
"The actor forgot to read the script and just made it up as he went along."
"I like when the actor improvises because he makes it feel real."
俳優が台本に頼らずに思う通りに言うことは、ad-libbing、improvising、またはmaking it up as they go alongと言えます。
例：
He began reading the script at first but then he began ad-libbing.
まず初めに彼は台本を読み始めたが、それからすぐアドリブを始めた。
When she is on stage she always improvises.
彼女がステージに立つと、いつも即興でやる。
The actor forgot to read the script and just made it up as he went along.
その俳優は、台本を読むのを忘れて即興で行った。
I like when the actor improvises because he makes it feel real.
その俳優が即興するのは好きです。何故ならば、リアルに感じさせてくれるからです。
"Play by ear" - To act and / or speak without first preparing for the act or the speech. - EX - I am going out for dinner tonight, but I don't know where. I will play it by ear.
"Unplanned" - To do something without a plan. - EX - I am going out for dinner tonight, but nothing is planned
"Play by ear" - 演技やスピーチをあらかじめ準備することなく、演技したり、話すこと
例文
- I am going out for dinner tonight, but I don't know where. I will play it by ear.
今晩夕食を食べに行くのだけど、どこにするか決めていないんだ。 行ってから決めよう
"Unplanned" - 計画なしに物事をすること
例文
- I am going out for dinner tonight, but nothing is planned
今晩夕食を食べに行くのだけど、何も決めていないんだ
Actors, musicians and story tellers do sometimes forget what is actually written in the script, song lyrics or story. During such instances, they 'ad lib'. 'Ad lib' is a verb that means saying something in place of the actual script (in the case of an actor) or the actual lyrics (in the case of a musician). Its present participle form 'ad libbing', can also be used as a noun.
So, you may say:
The actor / actress used ad libbing during the show because he/she forgot the script.
or
He/she ad libbed his/her way through the song because he/she forgot the lyrics
俳優やミュージシャン、ストーリーテラー（story teller）は、台本に書かれていること、歌詞、話を忘れることがあります。そういう状況で「ad lib」を言います。「ad lib（動詞）」は台本に書かれていること（＝俳優の場合）、歌詞と違うこと（＝歌手の場合）を言う」という意味です。
現在完了形の「ad libbing」は名詞としても使えます。
The actor / actress used ad libbing during the show because he/she forgot the script.
【訳】その俳優（女優）はセリフを忘れてアドリブを言いました。
He/she ad libbed his/her way through the song because he/she forgot the lyrics
【訳】彼（彼女）は歌詞を忘れて、アドリブで歌いました。
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