Lizzie, I was eagerly waiting for your opinion on this favorite subject of ours! And here you are, sharing such beautiful references. Thank you, dear.
The singer gives the soul and the actress gives the body,when both elements—voice and action, come together, the scene gains its charm and eternal depth. If either falters, the impact is reduced. As you mentioned, in many occasions we saw the difference.
Here are a few songs which are popular because of the singer's expressive voice---
Lata Mangeshkar: "Lag Ja Gale" Woh Kaun Thi?
Sadhana appeared graceful, but it was Lata's haunting voice that immortalised the song. Even without viewing the scene, audiences are moved by her rendition's agony, longing, and delicacy.
Mohammed Rafi, "Chaudhvin Ka Chand Ho"
Guru Dutt's expressions are subtle, but Rafi's voice lifts the tune to ethereal romance.
Kishore Kumar — "Kuch To Log Kahenge" Amar Prem
Sharmila Tagore's presence was understated, but Kishore's warm, philosophical rendition gave the song eternal emotional depth.
In the same way--- Here are a few examples of the actress' talent that added immense depth and dimension to the playback, making it memorable.
Madhubala: "Pyar Kiya To Darna Kya" Mughal-e Azam,
Lata's singing is faultless, but Madhubala's fierce defiance, eyes filled with love and rebellion, gave the scene legendary power!!
Meena Kumari: "Chalte Chalte Yunhi Koi" Pakeezah
The song is beautiful on its own, but Meena Kumari's sorrowful demeanour and restrained expressions make it heartbreaking!
Madhuri Dixit, "Maar Dala" in Devdas
Kavita Krishnamurthy's voice is impressive, but Madhuri's emotional movement and eyes lend the song grandeur and agony.
However, there are a few songs that highlight the singer's melodic voice and the actor's amazing talent, making them all-time greats.
The best examples are---
Tere Bina Zindagi Se Koi Shikwa-- Aandhi
Kishore-Lata's heartfelt duet combines perfectly with Sanjeev Kumar and Suchitra Sen's nuanced performances. Both the voice and the expression made it immortal.
"Tujhe Dekha To" Dilwale Dulhania Le Jayenge
Kumar Sanu and Lata Mangeshkar's passionate interpretation along with Shah Rukh-Kajol's chemistry resulted in one of Bollywood's most popular love ballads.
So, sometimes the voice alone keeps the song alive across generations, and occasionally the face on TV adds cultural significance. The great masterpieces are those in which both blend smoothly.
I agree with you on Govinda's songs....he brought energy, comic timing, quirky expressions, and effortless freestyle that made his songs unforgettable. 
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