Is it common for seiyū's to also be singers? While listening to the Magical Girl Lyrical Nanoha Openings I noticed that the first 4 Opening songs (Original Series - StrikerS) were all sung by Nana Mizuki, the seiyū of Fate Testarossa. Looking at her Wikipedia Page I've also noticed that she's also been the seiyū for Hinata Hyuga in the Naruto Movies.

Now the other groups I know of who did Openings/Endings are FLOW, ALI PROJECT, ORANGE RANGE, Kalafina and ClariS, but I do not know if any singer of these groups were also a seiyū for any series (particularly ones their music was in) but I am wondering if it is common for a seiyū to also be a singer? 

If so, is it also expected that if a singer sung the Opening/Ending Theme (or Insert Songs) they will also score a character role in the series?
For the first question, the answer is yes. This is especially true with female seiyuu. As you noted, Nana Mizuki is also a singer other than a seiyuu. You can check her discography here. She even went to a duet with T.M. Revolution. Other female seiyuu's that is worth noted:


Itou Kanae (Meat - Haganai)

Albums including Kokoro Keshiki and Miageta Keshiki
Hayami Saori (Shiraishi Urara - Yamada-kun to 7-nin Majo)

Released her first single August 2015
Kitamura Eri (Honoka Sisters - Sidonia no Kishi)

Has lot of singles and album.
Toyosaki Aki (Kanzaki Uiharu - Railgun)

Discography here
Tomatsu Haruka (Yuuki Asuna - SAO)

Discography here
Uchida Maaya (Takanashi Rikka - Chuunibyou)

Discography here
Nitta Emi (Kousaka Honoka - Love Live)

Discography here
PILE (Nishikino Maki - Love Live)

Was already a singer even before debut voicing as Maki.
Uchida Aya (Kotori - Love Live)

Example single is Spicaterrible
Nanjo Yoshino (Ayase Eri - Love Live, fripSide's Nidaime-sama

She was chosen to as fripSide's main vocalist after the first quit the group and go solo. First vocalist was nao who voiced 5pb in Hyperdimension Neptunia. She also released a mini-album and singles in ballad style which is different that her usual trance fripSide singing style.
Hirano Aya

She even own her own band with whom she got caught in a sex scandal.
Hanazawa Kana

Discography here
Sato Satomi

Discography here
Horie Yui

As pointed by Steven, Horie Yui also owned her own singles and albums.


It is more rare for male seiyuus to also be a singer although they do sing as the characters they voiced. Some that I remember are:


Tetsuya Kakihara (Natsu Dragneel - Fairy Tail)

Other than singing as Natsu, also released his own singles.
Ono Daisuke (Sebastian Michaelis - Kuroshitsuji)

Discography here.
Kenichi Suzumura (Zack Fair - Final Fantasy VII)

Discography here.
Morita Masakazu (Kurosaki Ichigo - Bleach)

His songs are mostly Bleach songs, that is he sang as Kurosaki Ichigo. As far as I know, Morita doesn't own his own singles/albums.
Hayami Shou (Aizen Sousuke - Bleach)

Other than singing as Aizen, also released some singles and albums.


For the second question, no. In fact, usually it is not. Only some anime have the singer of OP or ED also fills a role in the anime. Like Kitamura Eri (Honoka Sisters) sings Sidonia no Kishi's ED. But most of the time they don't. This is because a lot of anime still have bands and people who's primary job is singer perform the OP and ED. The anime Black Bullet's OP was performed by fripSide, but Nanjo-sama didn't have any role in the anime, nor Satoshi (fripSide's composer and part of the duo and he is also not a seiyuu).

As an extra, Hyde of L'Arc~en~Ciel and VAMPS also voiced an anime once as himself. But his main profession is a singer and not a seiyuu.