Music was one of the main comforts for Goodrem during the chemotherapy for Hodgkin's lymphomacancer diagnosis. Songs kept going around in her head: harmonies, melodies, lyrics, many deeper and darker than those that were on her debut album, Innocent Eyes. Goodrem states "It's a more grown up album", "My music changed because my life had changed; the whole world around me had changed. My music had to reflect that". "Extraordinary Day" is about the day (8 July 2003) when she was diagnosed with Hodgkin's lymphoma, "The Last Night On Earth" is a song about death and love, "The Analyst" tells the story of the hours of self-analysis she dealt with, "Be Strong" is an encouraging call for confidence and "Mistaken Identity" is Goodrem stating for the record where her head was at. She also stated "There are a lot of lyrics I wouldn't have used on the first album".
"In "Mistaken Identity" I sing 'I played the role of the nice girl next door who gets cut like a knife', I mention 'emotional suicide' in "Electric Storm": there's some intense lyrics in there. It was an intense year. I mean, it wouldn't be right if I didn't acknowledge what went on; everyone knows what went on. I feel like I know people personally even though I don't, but I feel I had to go into depth; get things out of the way, go through the journey and end with "You Are My Rock", which is a thank you song."
Musically the album is more classically piano driven and ventures into pop rock and pop jazz. The single "A Little Too Late" is guitar-driven pop rock song; "Mistaken Identity" is a dark, theatrical alternative pop song; while "The Last Night On Earth" is a pop rockballad.
Goodrem states that "This time around I had time to think `What kind of song do I want to create here?'", "I really felt the sky was the limit with this record, you could do anything in a song. I didn't feel limited at all. There were a lot more options, a lot more paths I could go down, more directions I could choose". Songwriter Guy Chambers had been keeping an eye on Goodrem's career and was impressed with her vocals and songwriting skills. The two writing together resulted in many of the album's key tracks. "'It's really unusual to work with somebody so talented, young, and brave. Who's not scared to take risks and who has the most emotional voice in pop music", You cannot help but believe every word Delta sings." Chambers states about working with Goodrem. Goodrem stated that Chambers was a classy producer and that he was an inspiration to work with. She felt they had a good connection on a musical level. "There's a definite story in the tracklisting," Goodrem says. "I felt I had to be really honest. Songs that people could still relate to but songs that also confronted issues. This past chapter of my life has been such an intense one and I wanted people to know my thoughts".
"Out of the Blue" was the first song off the album to be released and was released to radio on 24 September 2004 and quickly became the most added song to radio for that week. The music video was directed by Nigel Dick and made its premier to television on 1 October, [2004, right after the Australian soap operaNeighbours. Goodrem performed the song live for the first time on 17 October 2004 at the eighteenth Annual ARIA Awards and that same night "Out of the Blue" made its debut in the charts at number-one, Goodrem's sixth consecutive number-one single in Australia. Debuting at number-one on its first week, the song also was certified platinum by ARIA and stayed at number-one for two more weeks. The song also reached the top ten in the UK and the top twenty in Greece,Ireland, and New Zealand.
Mistaken Identity debuted at number one on the ARIA Album Chart on 15 November 2004 with triple platinum sales knocking Robbie Williams album Greatest Hits off the top spot. Unlike her debut album Innocent Eyes, Mistaken Identity only spent one week at the top, being knocked off by Eminem's album Encore, and falling to number two. In its third week, it fell to number three but in its fourth week it jumped up to number two. The album spent eight weeks in the top ten. When the album was in its fourth week in the chart and its position at number two, it had gone four times platinum and by its sixth week in the chart at number five it went five times platinum. The album spent forty-six weeks in the charts, leaving at number one hundred. It was the thirteenth-highest-selling album in Australia for 2004 and the thirtieth highest selling album for 2005. In 2005 the album was nominated for one ARIA Award, "Highest Selling Album" but lost to Missy Higgins album The Sound of White (2004). The album did not perform as well in the UK, debuting and peaking at number twenty-five and then falling to forty-four in its second week. It spent a total of ten weeks in the top seventy-five and accredited gold by BPI.
The second song lifted off the album was an Australian-only release, the title track "Mistaken Identity", released to radio on 6 December 2004, and became the second-most-added song to radio for that week. The music video for the song was directed by Michael Spiccia, released on 13 December 2004 and was based on the 1984 film The NeverEnding Story. It was released as a CD single in January 2005 and debuted at number seven on the Australian Singles Chart, the fifth highest entry for that week and was certified gold by ARIA. "Almost Here" was the third song released from the album but was the second internationally, released on 31 January 2005. A song about a broken relationship, the video was filmed in a London airport between 27 October to 29 October 2004. It became one of Goodrem's biggest hits, making it her seventh number-one single in Australia, her first in Ireland, and her fifth top ten hit in the UK, and was nominated for an ARIA Award in 2005 for "Highest Selling Single" but lost to "The Prayer" by Anthony Callea. "A Little Too Late" was the fourth song released from the album on 27 May 2005 and the music video was directed by MTV award-winner Anthony Rose and was premiered 2 May 2005 on the Channel Ten news. The song was only released in Australia and did not chart like the other singles, debuting and peaking at number thirteen. The last song released from the album was "Be Strong", which was only released in Australia as a digital download on 16 October 2005.