Lord Visnu, by His mystic power, would assume a form that would captivate even Lord Siva. Therefore Lord Visnu was grave and at the same time was smiling

Expressions researched:
"Lord Viṣṇu, by His mystic power, would assume a form that would captivate even Lord Śiva. Therefore Lord Viṣṇu was grave and at the same time was smiling"

Srimad-Bhagavatam

SB Canto 8

The illusory, material energy is represented by Durgādevī, who is the wife of Girīśa, or Lord Śiva. Durgādevī could not captivate Lord Śiva's mind, but now that Lord Śiva wanted to see Lord Viṣṇu's feminine form, Lord Viṣṇu, by His mystic power, would assume a form that would captivate even Lord Śiva. Therefore Lord Viṣṇu was grave and at the same time was smiling.

SB 8.12.14, Translation and Purport

Śukadeva Gosvāmī said: When Lord Viṣṇu was thus requested by Lord Śiva, who carries a trident in his hand, He smiled with gravity and replied to Lord Śiva as follows.

The Supreme Personality of Godhead, Viṣṇu, is known as Yogeśvara. Yatra yogeśvaraḥ kṛṣṇaḥ (BG 18.78). Mystic yogīs want to acquire some power by practicing the yoga system, but Kṛṣṇa, the Supreme Personality of Godhead, is known as the Supreme Lord of all mystic power. Lord Śiva wanted to see the Mohinī-mūrti, which was captivating the entire world, and Lord Viṣṇu was gravely thinking of how to captivate Lord Śiva also. Therefore the word bhāva-gambhīram is used here. The illusory, material energy is represented by Durgādevī, who is the wife of Girīśa, or Lord Śiva. Durgādevī could not captivate Lord Śiva's mind, but now that Lord Śiva wanted to see Lord Viṣṇu's feminine form, Lord Viṣṇu, by His mystic power, would assume a form that would captivate even Lord Śiva. Therefore Lord Viṣṇu was grave and at the same time was smiling.